The present invention generally relates to an internal combustion engine and more particularly, to an air intake arrangement of a Diesel engine for use, for example, in a motor vehicle or the like.
Commonly, a Diesel engine, different from a gasoline engine, is not provided with a throttle valve in its air intake passage, and therefore, the amount of intake air becomes excessively large during light-load operation, especially in the case of engine idling, resulting in an increase of compression resistance of the engine, with torque variation tending to be produced. Accordingly, there is presented a problem that the engine is caused to vibrate due to the torque variations mentioned above, giving rise to engine noises resulting from the vibrations.
In order to overcome the disadvantages as described above, there has conventionally been proposed, for example, in U.S. Pat. No. 4,058,101, a control apparatus for Diesel engine in which an intake air shutter valve equivalent to a throttle valve is provided in an intake air passage for selective closing and opening of the intake air passage so as to close the intake air shutter valve only during the light-load operation (i.e. to reduce the effective passage area of the intake air passage) so that the amount of engine vibrations and vibration noises are decreased through reduction of the amount of intake air.
In order to confirm the effectiveness of the arrangement in said U.S. Pat. No. 4,058,101, the present inventors first carried out a series of experiments to investigate the relation between the amount of throttling of the intake air shutter valve and the amount of engine vibrations after engine warm-up had been completed (i.e. during the period after the engine has warmed up). In the above experiments, the amount of the throttling of the intake air shutter valve was represented by the intake air negative pressure downstream of the intake air shutter valve during the idling operation, i.e. when the engine was running at 600 r.p.m. It is to be noted that the amount of throttling of the intake air shutter valve will be represented hereinbelow in the similar manner to the above. Meanwhile, since the transmission case is also subjected to vibrations in a similar manner as the engine is caused to vibrate, and thus, the amount of engine vibrations may be regarded as equal to the amount of vibrations of the transmission case, the relation between the amount of throttling of the intake air shutter valve and the amount of transmission case vibrations was also studied in the experiments with respect to the warmed-up engine under the state of idling operation, the results of which are shown in FIG. 2. It is needless to say that noises due to the vibrations show a trend similar to the amount of vibrations of the transmission case.
Subsequently, upon review of the results of the experiments as described above, it is seen that the amount of vibrations of the transmission case is gradually decreased as the amount of throttling of the intake air shutter valve is increased from the fully opened state thereof (throttling amount of 3 mmHg). Therefore, it is ensured that the engine vibrations and noises due to the vibrations may be reduced with an increase in the amount of throttling of the intake air shutter valve.
Furthermore, upon study into the amount of throttling of the intake air shutter valve, it is noticed that the amount of vibrations of the transmission case (i.e. the amount of engine vibrations) is reduced by about 3 dB, or vibration energy is reduced to approximately 1/2, when the amount of throttling is increased by 100 mmHg with respect to the fully opened state of the intake air shutter valve. Additionally, it has also been confirmed that the reduction of the vibration energy to such an extent, i.e. reduction of the amount of vibration of the transmission case to below approximately 98 dB, is necessary.
On the other hand, when the amount of throttling exceeds 400 mmHg, the amount of intake air is extremely reduced, with consequent deterioration of the state of combustion, resulting in such problems as the occurrence of semi-misfire, the discharging of white smoke and the creation of the odor of formaldehyde.
As a result of the foregoing studies, has been found that it is desirable that the amount of throttling of the intake air shutter valve be in the region between 100 mmHg and 400 mmHg during idling operation at the completion of engine warm-up, and more preferably, in the range between 150 mmHg and 350 mmHg.
However, when the engine is in a cold state, if the amount of throttling of the intake air shutter valve is set to be above 100 mmHg in a similar manner as at the completion of engine warm-up described above even during the idling operation, the state of combustion is deteriorated as in the throttling to more than 400 mmHg upon completion of engine warming-up mentioned earlier, resulting in such inconveniences as semi-misfire, generation of white smoke and odor of formaldehyde, etc.
Accordingly, when the engine is in the cold state, it becomes necessary to reduce the amount of throttling of the intake air shutter as compared with that in the warmed-up state of the engine so as to increase the amount of intake air in order to improve the state of combustion.
Incidentally, some of the motor vehicles equipped with Diesel engines employ exhaust brakes, which include an exhaust brake valve operably provided in an exhaust passage of the engine for braking the motor vehicle by causing compression loss to take place through an increase of exhaust gas pressure within the exhaust passage upon closing of the exhaust brake valve, i.e. through an increase of compression resistance of the engine by raising the engine back pressure.
However, where the exhaust brake as described above is employed, noises or intake air noises are developed by rapid expansion of gas compressed within the engine and flowing backward into the intake air passage. For overcoming the above disadvantage, there has also been conventionally proposed an exhaust brake device, described for example, in Japanese Utility Model Publication Jikkosho No. 43-14326, in which the intake air passage is arranged to be throttled by closing the intake air shutter valve for reducing the passage area of the intake air passage even during use of the exhaust brake.
In the above known arrangement, the intake air shutter valve may be closed even during speed retardation as well as in the idling operation, but, since it is necessary to supply more than a predetermined amount of intake air into the combustion chamber in order to achieve sufficient functioning of the exhaust brake, when the reduction of the intake air noises during actuation of the exhaust brake is intended by operating the intake air shutter valve even during the functioning of the exhaust brake, the amount of throttling of the intake air shutter device can not be increased to a large extent, because upon reduction of the intake air amount supplied into the combustion chamber by increasing the amount of throttling of the intake air shutter device (i.e. the degree of closing of the intake air shutter valve), the pressure in the combustion chamber becomes low, even if discharging of the exhaust gas is suppressed by the exhaust brake valve, with the compression ratio remaining large, and thus, it is not possible to subject the engine to a sufficiently large compression loss.
Therefore, for reducing the intake air noises, while maintaining braking performance of the exhaust brake in a favorable state during speed retardation, with the exhaust brake functioning, it is surmised that there is a preferable range of throttling of the intake air shutter valve.
According to the experiments, it is desirable that the amount of throttling of the intake air shutter valve be less than 70 mmHg from the viewpoint of brake performance, while the intake air noises are suppressed more effectively with the increase of throttling of the intake air passage, and it has been found by the experiments that the noise energy is reduced by more than 1/2 in the case where the amount of throttling of the intake air shutter valve is more than 15 mmHg as compared with the case in which no intake air shutter valve is provided in the intake air passage. Thus, it is seen that, during speed retardation in which the exhaust brake is operated, the amount of throttling of the intake air shutter valve should preferably be in the range between 15 and 70 mmHg from the viewpoints of the braking performance and reduction of intake air noises.
When the foregoing results of studies are synthetically taken into consideration, it is understood that after the engine has warmed up, if the amount of throttling of the intake air shutter valve is set to be more than 100 mmHg, and more preferably, in the range of 150 mmHg to 350 mmHg in the light-load period, and also set to be in the range of 15 mmHg to 70 mmHg in the reduced speed period, the exhaust brake functions effectively, with simultaneous elimination of vibrations and noises of the engine.